An iron-based superconductor was discovered early in 2008 (non-patent document 1). Since this discovery of the iron-based superconductor, superconductors of the related compounds have been successively found, and the iron-based superconductor is expected to be a vein of new high-temperature superconductor.
The iron-based superconductors have a high critical current density and a high critical magnetic field, and are increasingly expected to be applied to practical use. Among the iron-based superconductors, ones having a simplest structure, such as FeSe, FeTe, FeSe1-xTex (0<x<1), and FeTe1-xSx (0<x<1), have been developed by the present inventors (for example, non-patent documents 2 and 3). Further, it has been known that, in the molar ratio of Fe and a chalcogen in the compound, Fe is slightly excess. These iron-based superconductors are considered suitable for practical application due to their relatively low toxicity or simple structure.    [Non-patent document 1] J. Am. Chem. Soc., 130, 3296 (2008)    [Non-patent document 2] Appl. Phys. Lett, 94, 012503 (2009)    [Non-patent document 3] Appl. Phys. Lett, 93, 152505 (2008)